In a conventional construction of a high intensity discharge lamp, the electrode arrangement is hermetically sealed to the polycrystalline alumina arc tube by a glass frit with specific composition to match the thermal expansion coefficient of the polycrystalline alumina arc tube. In making the electrode, materials such as niobium metal, molybdenum-alumina cermet, or tungsten-alumina cermet are used since their thermal expansion coefficients are close to that of the polycrystalline alumina. Even with the careful design of the sealing frit material, cracking failure in the sealing area during lamp manufacture and lamp life cannot be completely prevented due to the construction of the electrode. In most electrode designs, there is molybdenum coil encircling either a molybdenum rod or a tungsten rod disposed between the frit sealing area and the tungsten electrode tip. When the frit over flows onto this middle portion of the electrode during the sealing process, there is a possibility of cracking in the sealing area during the sealing process or during lamp life.
Furthermore, due to the difference of thermal expansion coefficient between polycrystalline alumna and molybdenum, a relatively larger gap exists between the inner diameter of the polycrystalline alumna capillary tube and the molybdenum coil. This gap plus the void space between the molybdenum coil turns require that more metal halide chemical fill amount be filled into the arc tube during arc tube manufacturing. Higher amounts of metal halide chemical fill will introduce more impurity into the arc tube causing starting problems and increasing the rate of chemical reaction with polycrystalline alumina material. In order to reliably prevent cracks due to thermal expansion coefficient mismatch in the sealing region, reduce metal halide fill amount and improve lamp performance, an improved electrode arrangement is proposed.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.